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Breathing in the city

technical & practice: One passive approach to improving urban indoor air quality is to draw air from the highest points of buildings BY NEIL SPILLER

Living in any city is fraught with unseen health risks. One of the chief villains is poor air quality. But how can we mitigate its effects when designing ventilation systems for urban buildings?

The air acts as a carrier for particulate matter, gaseous pollution and noise pollution. Concentrations of pollutants, especially fine particulates, are often found in urban canyons. Pollution levels can vary considerably from street to street and with height as wind-driven air contorts itself to cope with hot spots and the varied topology of the city. In the face of the vortices and pressure differentials that are created, it seems logical to site air intake terminals as high as possible.

Stephen Gage, who is the Bartlett's new Professor of Innovative Technology, is one of a research team funded by detr which has built large test installations to research top-down ventilation of buildings in urban areas. The team's installations have been tested in relation to roof-intake mechanisms, summer cooling, winter heating, reliability in non-typical conditions and other criteria*.

First, the wind needs to be caught. Traditional wind catchers are mostly fixed scoops oriented to face a prevailing wind. These become ineffective if the wind direction changes, as it constantly does, in complex urban areas. Another traditional, and modern, form is the split-duct wind catcher with four or more segments pointing in different directions. Wind creates positive and negative pressure in the windward and leeward sides of the terminal respectively, allowing it to function independently of fluctuations in wind direction.

When conditions are calm, wind catchers can stop working if they are serving otherwise sealed spaces. Traditional split-duct systems found in the Middle East serve spaces which have other openings to the outdoors. Wind catchers therefore also provide ventilation by stack effect (under another name, 'gravity') in calm conditions. The Bartlett team decided that in order to ventilate spaces without the use of fans, its systems must also utilise such gravity-displacement techniques.

Gravity displacement is inherent in all passive-ventilation systems. The stack-effect pressure is dependent on the temperature difference between inside and outside air and the height of the air column. These temperatures vary considerably over a yearly, seasonal and diurnal cycle.

On summer nights, cooling can be achieved in a passively ventilated building by passing large quantities of air though the spaces to cool the structure. A top-down gravity displacement system will ventilate at night when external temperatures are lower than internal ones. Gravity will drive the cooler air down the duct. But in the summer daytime, if the internal temperature has fallen to lower than the external air temperature, an additional means is needed to drive air flow down the duct in order to provide fresh ventilation air. If fans are to be avoided, air in the intake duct must be cooled until its temperature is below that of the internal air, and the air in the extract duct can also be heated to help induce air flow.

It might be possible to use solar-powered heat pumps for this, transferring heat from inlet to outlet air streams in summer. Cooling the inlet air has the added benefit of introducing cooled air at low velocity to the occupied space.

Top-down ventilation does not prohibit multi-storey buildings. In the uk it might work up to about six storeys. The practical limit comes from the increasing cross section of ducts as building height increases; thus the increasing fraction of floor area that ducts take up. With airspeed in ducts rarely getting above 1m/s, ducts are relatively large compared with those of mechanical air systems.

There are a few significant design issues for multi-storey buildings. Thermal insulation must be included at each internal floor level, otherwise heat rising to ceiling level on one floor will warm the floor above, and so on progressively up the building. Variable dampers and temperature sensors will be required to regulate air supply. Fire spread via vertical ducts and sound transmission though the building also need to be considered.

Terminal design

Another crucial factor in the design of top-down ventilation systems is the form of the intake and extract terminals. These need to be larger than the ducts they serve to reduce the risk of insect or rain entry. Extract terminals have been shown to function best when 'H' pots or just plain vertical pots (still larger in diameter than the ducts) are used.

The Bartlett built six full-scale, wind-assisted installations. In terms of intake terminals, the optimum strategy seemed to be to present an open area to the wind. Intakes can be shaded to limit their heating up in the summer, perhaps with an insulated cap. Intakes should either rotate to face the wind, or adjustable dampers can be opened or closed to adapt static split-duct wind catchers to variable wind directions and to permit displacement ventilation.

The Bartlett team designed five alternative composite wind catchers, that is, with combined intake and extract in a single fitting, including fixed and rotary designs. Their performance has been evaluated in different wind conditions.

Ideal applications for these types of installations are the large spaces in educational buildings, open-plan offices, shopping centres, museums and galleries, concert halls, theatres and restaurants. Such buildings will also include small cellular spaces that require natural ventilation and natural light. Ducting air from stacks to these spaces is not viable because driving pressures are too low. But top-down ventilation systems will work well for these in conjunction with direct exterior access (such as to urban gardens, internal courtyards, etc). To avoid polluting inlet air, vehicles should be routed elsewhere. Planning restrictions should also be placed on 'back-land' parking generally. And intake/extract towers should be excluded from normal planning height restrictions.

We are still at the beginning of research into the provision of cleaner, greener, urban areas and buildings. It is clear that creative opportunities inherent in these notions can providing architectural elements which mitigate the fear many architects have that green means boring.

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The Architects' Journal is the voice of architecture in Britain. We sit at the heart of the debate about British architecture and British cities, and form opinions across the whole construction industry on design-related matters